The red background with white lettered sign at the entrance to the St. Thomas University. Welcome to St Thomas University – a step above UNB!
The red background to the STU entrance sign is an igneous rock, as is the rock seen at Stop 1 (the boulder outside the geology building at UNB), though the grain size is larger and one of the principle minerals here is the pink-red mineral ‘orthoclase’, compared with the white ‘plagioclase’ seen dominating the glacial boulder. The abundant clear, glassy mineral here is quartz. It is the grain size and the relative proportions of these minerals that allow geologists to discern one rock type from another and assign them names.
EXERCISE: Using the Tables below, your observations of the crystal size and the facts presented above, what name would you assign to the rock seen in the sign?
ORTHOCLASE RICH ROCKS (GENERALLY PINK) - If the rock is pink, use this table.
No quartz
Minor quartz
Abundant quartz
Coarse grained
Alkali gabbro
Syenite
Granite
Medium grained
Alkali microgabbro
Microsyenite
Microgranite
Fine grained
Alkali basalt
Trachyte
Rhyolite
PLAGIOCLASE RICH ROCKS (GENERALLY WHITISH-GREY) - If the rock is grey-whitish, use this table.
No quartz
Minor quartz
Abundant quartz
Coarse grained
Gabbro
Diorite
Granodiorite
Medium grained
Diabase
Microdiorite
Microgranodiorite
Fine grained
Basalt
Andesite
Dacite
(NB. Coarse grained is assigned to rocks with crystals > 3mm diameter, Medium grained to crystal sizes of 1-3 mm, and < 1mm grain sizes are considered fine grained.)
St Thomas University Sign
The red background with white lettered sign at the entrance to the St. Thomas University. Welcome to St Thomas University – a step above UNB!
The red background to the STU entrance sign is an igneous rock, as is the rock seen at Stop 1 (the boulder outside the geology building at UNB), though the grain size is larger and one of the principle minerals here is the pink-red mineral ‘orthoclase’, compared with the white ‘plagioclase’ seen dominating the glacial boulder. The abundant clear, glassy mineral here is quartz. It is the grain size and the relative proportions of these minerals that allow geologists to discern one rock type from another and assign them names.
EXERCISE: Using the Tables below, your observations of the crystal size and the facts presented above, what name would you assign to the rock seen in the sign?
ORTHOCLASE RICH ROCKS (GENERALLY PINK) - If the rock is pink, use this table.
PLAGIOCLASE RICH ROCKS (GENERALLY WHITISH-GREY) - If the rock is grey-whitish, use this table.